Entity-Relationship Model Lecture 5

Database Modeling and Implementation Process Ideas ER Design Relational Schema Relational DBMS Implementation

ER Model Components Entity Sets Attributes Relationships

ER Model The enterprise data can be described as a set of entities and a set of relationships Entity – Data that describes some component of the enterprise. Example: specific person, specific auto are entities. An entity is characterized by its properties (attributes) An Entity Set is a set of entities that are characterized by the same set of attributes. Example: a set of persons, set of autos.

Domain – the set of permitted values for each attribute Attributes An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set. Domain – the set of permitted values for each attribute Attribute types: Simple and composite attributes. Single-valued and multi-valued attributes Derived attributes Can be computed from other attributes student = (student-id, student-name, student-street, student-city)

Composite Attributes MultiValued Attributes: Attribute that has more than one value: set of telephone numbers set of addresses in : 1 Person 2 Name 2 telephone# 2 address

Derived Attributes Attribute whose value is derived from other attributes 1 Person 2 Name 2 telephone# 2 address 2 DOB 2 Age derived from DOB and current date

A relationship is a cartesian product of n  2 entities {(e1, e2, … en) | e1  E1, e2  E2, …, en  En} where (e1, e2, …, en) is a relationship

Relationship Set borrower

Relationship Sets An attribute can also be property of a relationship set.

Degree of a Relationship Set Refers to number of entity sets that participate in a relationship set. Relationship sets that involve two entity sets are binary (or degree two). Generally, most relationship sets in a database system are binary. Relationship sets may involve more than two entity sets. Relationships between more than two entity sets are rare. Most relationships are binary. E.g. Suppose employees of a bank may have jobs (responsibilities) at multiple branches, with different jobs at different branches. Then there is a ternary relationship set between entity sets employee, job and branch

Courses Enrolls Students Instructors Students Courses TAs Ann CS43005 Jan Sue CS43005 Pat Bob CS43005 Jan … … …

ER Diagrams - Symbols

Types of Binary Relationships Many-many Many-one One-one Representation of Many-One Many-one E/R: arrow pointing to “one.”

One-To-One Relationships Put arrows in both directions. Design Issue: Here, manufacturer is an Entity Set. In earlier diagrams it is an attribute. Which is right? Best- seller Manfs Beers

One-To-Many Relationship In the one-to-many relationship a loan is associated with at most one customer via borrower, a customer is associated with several (including 0) loans via borrower

Many-To-One Relationships In a many-to-one relationship a loan is associated with several (including 0) customers via borrower, a customer is associated with at most one loan via borrower

Participation of an Entity Set in a Relationship Set Total participation (indicated by double line): every entity in the entity set participates in at least one relationship in the relationship set E.g. participation of loan in borrower is total every loan must have a customer associated to it via borrower Partial participation: some entities may not participate in any relationship in the relationship set E.g. participation of customer in borrower is partial

Many-To-Many Relationship A customer is associated with several (possibly 0) loans via borrower A loan is associated with several (possibly 0) customers via borrower

Keys for Relationship Sets The combination of primary keys of the participating entity sets forms a super key of a relationship set. (customer-id, account-number) is the super key of depositor NOTE: this means a pair of entity sets can have at most one relationship in a particular relationship set. E.g. if we wish to track all access-dates to each account by each customer, we cannot assume a relationship for each access. We can use a multivalued attribute though Must consider the mapping cardinality of the relationship set when deciding the what are the candidate keys Need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key

Beers-Bars-Drinkers Example Our running example. name addr license Frequents Serves Bars Likes Beers Drinkers name manf name addr

Example: Drinkers Have Favorite Beers name addr license Frequents Serves Bars Likes Beers Drinkers Favorite name manf name addr

Attributes on Relationships price Sells Bars Beers Shorthand for 3-way relationship: price Prices Sells Bars Beers

A true 3-way relationship. Price depends jointly on beer and bar. Notice arrow convention for multiway relationships: “all other E.S. determine one of these.” Not sufficiently general to express any possibility. However, if price, say, depended only on the beer, then we could use two 2-way relationships: price-beer and beer-bar. Or better: just make price an attribute of beer.

Converting Multiway to 2-Way Create a new connecting E.S. to represent rows of a relationship set. E.g., (Joe's Bar, Bud, $2.50) for the Sells relationship. Many-one relationships from the connecting E.S. to the others. BBP The- Bar The- Beer The- Price Bars Beers Price

Converting Multiway to 2-Way Stars Contract Movies Studio_Producer Studio_star Studios

Converting Multiway to 2-Way Stars Movies movie_of Star-_of Contracts Studio Producer Studio_star Studios

Summary of Symbols

Roles Entity sets of a relationship need not be distinct The labels “manager” and “worker” are called roles; they specify how employee entities interact via the works-for relationship set. Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles. Role labels are optional, and are used to clarify semantics of the relationship

Specialization within an entity set that are distinctive from other entities in the set. These subgroupings become lower-level entity sets that have attributes or participate in relationships that do not apply to the higher-level entity set. Depicted by a triangle component labeled ISA (E.g. Top-down design process; we designate subgroupings customer “is a” person). Attribute inheritance – a lower-level entity set inherits all the attributes and relationship participation of the higher-level entity set to which it is linked.

Specialization Example

Generalization A bottom-up design process – combine a number of entity sets that share the same features into a higher-level entity set. Specialization and generalization are simple inversions of each other; they are represented in an E-R diagram in the same way. The terms specialization and generalization are used interchangeably.

Aggregation Consider the ternary relationship works-on, which we saw earlier Suppose we want to record managers for tasks performed by an employee at a branch

E-R Diagram With Aggregation

Existence Dependencies If the existence of entity x depends on the existence of entity y, then x is said to be existence dependent on y. y is a dominant entity (in example below, loan) x is a subordinate entity (in example below, payment) loan-payment payment loan If a loan entity is deleted, then all its associated payment entities must be deleted also.

Weak Entity Sets An entity set that does not have a primary key is referred to as a weak entity set. The existence of a weak entity set depends on the existence of a identifying entity set it must relate to the identifying entity set via a total, one-to-many relationship set from the identifying to the weak entity set Identifying relationship depicted using a double diamond The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set. The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set’s discriminator.

Weak Entity Sets We depict a weak entity set by double rectangles. We underline the discriminator of a weak entity set with a dashed line. payment-number – discriminator of the payment entity set Primary key for payment – (loan-number, payment-number)

Example student instructor enrolls teaches offering isoffered course requires

Design Issues Use of entity sets vs. attributes Choice mainly depends on the structure of the enterprise being modeled, and on the semantics associated with the attribute in question. Use of entity sets vs. relationship sets Possible guideline is to designate a relationship set to describe an action that occurs between entities Binary versus n-ary relationship sets Although it is possible to replace any nonbinary (n-ary, for n > 2) relationship set by a number of distinct binary relationship sets, a n-ary relationship set shows more clearly that several entities participate in a single relationship. Placement of relationship attributes

Design Choices Avoid introducing more elements into your design than necessary Do not introduce all possible relationships but only necessary ones. Try to avoid too many entities. Replace them with attributes if possible

E-R Diagram for a Banking Enterprise

Reduction of an E-R Schema to Tables Primary keys allow entity sets and relationship sets to be expressed uniformly as tables which represent the contents of the database. A database which conforms to an E-R diagram can be represented by a collection of tables. For each entity set and relationship set there is a unique table which is assigned the name of the corresponding entity set or relationship set. Each table has a number of columns (generally corresponding to attributes), which have unique names.

Representing Entity Sets as Tables A strong entity set reduces to a table with the same attributes.

Composite and Multivalued Attributes Composite attributes are flattened out by creating a separate attribute for each component attribute E.g. given entity set customer with composite attribute name with component attributes first-name and last-name the table corresponding to the entity set has two attributes name.first-name and name.last-name A multivalued attribute M of an entity E is represented by a separate table EM Table EM has attributes corresponding to the primary key of E and an attribute corresponding to multivalued attribute M E.g. Multivalued attribute dependent-names of employee is represented by a table employee-dependent-names( employee-id, dname) Each value of the multivalued attribute maps to a separate row of the table EM

Representing Relationship Sets as Tables A many-to-many relationship set is represented as a table with columns for the primary keys of the two participating entity sets, and any descriptive attributes of the relationship set. E.g.: table for relationship set borrower

Additional Rules for Translating Relationship into Relation If one entity set participates several times in the relationship with different roles, its key attributes must be listed as many times and with different names for each role. Studies(SSN, Name); Favorite(SSN, Name); Friends(SSN1, SSN2) Name SSN studies subject Student friends favorite

Redundancy of Tables Many-to-one relationship sets that are total on the many-side can be represented by adding an extra attribute to the many side, containing the primary key of the one side Example: We eliminate relation Favorite and we extend relation for Student as follows: Student(SSN, Name, Subject.name) If, however, the relationship is many-to-many we cannot do that since it leads to redundancy For example relation Studies cannot be eliminated since otherwise we may end up with: 111-222-333 John OS 111-222-333 John DBMS

Representing Weak Entity Sets A weak entity set becomes a table that includes a column for the primary key of the identifying strong entity set

Representing Weak Entity Sets (Additional Rules) The relation for any relationship in which the weak W entity participates must use as a key for W all of its key attributes including those of strong entities that contribute to the W key Weak entity set W participating in the relationship should not be converted into a relation.

Representing Specialization as Tables Form a table for the higher level entity Form a table for each lower level entity set, include primary key of higher level entity set and local attributes table table attributes person name, street, city customer name, credit-rating employee name, salary Drawback: getting information about, e.g., employee requires accessing two tables

Relations Corresponding to Aggregation To represent aggregation, create a table containing primary key of the aggregated relationship, the primary key of the associated entity set Any descriptive attributes

Example booked departure assigned instantof canfly flight plane ssn name ISA person passenger age booked ISA date departure assigned pilot #fhrs gate instantof canfly flight plane dtime atime F# man model

Relational schema for the ER diagram Passenger(ssn) Passenger(ssn, f#, date) Departure(f#, date, gate) departure(f#,date,gate,man,model,ssn) Booked(f#, ssn) Flight(f#, dtime, atime) Flight(f#, dtime,atime) Assigned(f#, man, model, ssn) Person(ssn, name, age) Person(ssn,name,age) Pilot(ssn, #hrs) Pilot(ssn,#hrs,man,model,f#,date) Plane(man, model) Plane(man,model) Canfly(man, model, ssn)